ATLANTA — The second-largest extinction in the Earth’s history, the killing of two-thirds of all species, may have been caused by ultraviolet radiation from the sun after gamma rays destroyed the Earth’s ozone layer.

Astronomers are proposing that a supernova exploded within 10,000 light years of the Earth, destroying the chemistry of the atmosphere and allowing the sun’s ultraviolet rays to cook fragile, unprotected life forms.

All this happened some 440 million years ago and led to what is known as the Ordovician extinction, the second most severe of the planet’s five great periods of extinction.

“The prevailing theory for that extinction has been an ice age,” said Adrian L. Melott, a University of Kansas astronomer. “We think there is very good circumstantial evidence for a gamma ray burst.”

One-two punch
Melott is the leader of a team, which includes some astronomers from the National Aeronautics and Space Administration, that presented the theory Wednesday at the national meeting of the American Astronomical Society.

Data: The rise and fall of Earth's species
Fossil records for the Ordovician extinction show an abrupt disappearance of two-thirds of all species on the planet. Those records also show that an ice age that lasted more than a half million years started during the same period.

He said a gamma ray beam striking the Earth would break up molecules in the stratosphere, causing the formation of nitrous oxide and other chemicals that would destroy the ozone layer and shroud the planet in a brown smog.

“The sky would get brown, but there would be intense ultraviolet radiation from the sun striking the surface.” he said. The radiation would be at least 50 times above normal, powerful enough to killed exposed life.

In a second effect, the brown smog would cause the Earth to cool, triggering an ice age, Melott said.

The extinction “could have been a one-two punch,” said Bruce S. Lieberman, a paleontologist at the University of Kansas and a co-author of the theory. “Our theory builds on earlier theories” that included an ice age.

Answer for the ice age
Before the extinction, the Earth was unusually warm. Melott said climate experts have been unable to find a model that would explain the sudden onset of massive glaciers.

Jere H. Lipps, a paleobiologist at the University of California, Berkeley, said gamma rays as a source of the Ordovician extinction should be regarded as only one of several theories. “It is a hypothesis that should be tested,” Lipps said.

He said the widely-accepted idea that the dinosaurs were wiped out by an asteroid 65 million years ago started out as a “wild idea” but that it gained wide support after other research.

Most of the life killed in the Ordovician extinction were primitive sea creatures. Those that lived at or near the surface would be greatest risk from the ultraviolet radiation. Melott the species killed lived in shallow waters or reproduced with larvae that spent part of their lives near the water surface. Animals living in deep water were not harmed.

There were only primitive plants living on land, but they, too, would have been affected, he said.

Earth hit more than once
Melott said it is almost certain that Earth has been zapped by a gamma rays several times in its 4.5 billion year history.

“You can expect a dangerous gamma ray burst every few hundred million years,” he said. “It could happen tomorrow or it could be millions of years.”

Supernovae, the source of gamma rays, usually leave behind remnant clouds of dust, shock waves and black holes that can be detected for millions of years. Melott said there is no known evidence of such a nearby supernova, but that in 440 million years the Milky Way would have rotated almost twice and traces of the explosion could have been moved during that time.

The Ordovician was the first of five great extinctions in history.

The Devonian, 360 million years ago, killed 60 percent of all species; the Permian-Triassic, 250 million years ago, killed 90 percent of all life; the late Triassic, 220 million years ago, killed half of all species; and the Cretacious-Tertiary event destroyed the dinosaurs and half of all other species about 65 million years ago.